Nature is the ultimate nanofabricator. The latest evidence is an unusual fragment of ancient Roman glass (called “wow glass”) that boasts a thin gold patina. Roman glass shards are notable for their iridescent shades of blue, green and orange – the result of a corrosion process that slowly restructures the glass. photonic crystals– and the shimmering mirror-gold luster of this particular shard is a rare example with unusual optical properties, according to new paper published in the Proceedings of the National Academy of Sciences.
It is another example of naturally occurring structural coloration. As previously stated, the bright iridescent colors in butterfly wings, soap bubbles, opals, or beetle shells do not come from any pigment molecules, but from how they are structured—naturally occurring. photonic crystals. In nature, for example, chitin scales (a polysaccharide common to insects) are arranged like roof tiles. Basically, they form a diffraction gratingexcept that photonic crystals only produce specific colors or wavelengths of light, while a diffraction grating will produce the entire spectrum, much like a prism.
Photonic crystals, also known as band-gap photonic materials, are “tunable,” meaning they are precisely arranged to block certain wavelengths of light while letting others through. Change the structure by changing the size of the tiles and the crystals become sensitive to a different wavelength. They are used in optical communications as waveguides and switches, as well as in filters, lasers, mirrors and various anti-reflective stealth devices.
Scientists can make their own structural color materials in the lab, but it can be challenging to scale up the process for commercial applications without sacrificing optical precision. Creating structural colors similar to those in nature is therefore an active area of materials research. For example, at the beginning of this year, researchers from the University of Cambridge developed a new innovative plant-based film that cools when exposed to sunlight, making it ideal for future cooling of buildings or cars without the need for any external energy source. The films created are colored, but this is a structural color in the form of nanocrystals, not due to the addition of pigments or dyes.
And last year, researchers at the Massachusetts Institute of Technology adapted a 19th-century holographic photography technique invented by physicist Gabriel Lippmann to develop chameleon-like films that change color when stretched. The foils would be ideal for making bandages that change color in response to pressure to let doctors know if they are wrapping a wound too tightly – an important factor in treating conditions such as leg ulcers, pressure ulcers, lymphedema and scarring. Children would love to wear bandages that change color, which is a boon to pediatricians. And the ability to produce large sheets of material opens up applications in apparel and sportswear.
Fiorenzo Omenetto, a materials scientist at Tufts University who co-authored the new paper, noticed the unique shard while visiting the Italian Institute of Technology’s Center for Cultural Heritage Technology and decided it deserved further scientific study. “This beautiful sparkling piece of glass on the shelf caught our attention,” said Omenetto. “It was a fragment of Roman glass found near the ancient city of Aquileia in Italy. The director of the center nicknamed it “wow glass”.
Aquileia was founded by the Romans in 181 BC, initially as a military base, but soon flourished as a center for trade, including wrought metal, Baltic amber, wine and ancient glass. “The discovery of a wooden barrel containing 11,000 glass fragments in a Roman shipwreck in the sea waters off Aquileia demonstrates the city’s leadership in the exchange and processing of recycled glass along trade routes,” the authors wrote. In the second century AD, at its peak, the city had a population of 100,000. Its fortunes dwindled after it was sacked by Attila and his Huns in 452 and again by the Lombards in 590. Today, the town has only about 3,500 inhabitants, but it remains an important archaeological site.
Archaeologists found the “wow glass” in the topsoil of an agricultural field during a field walk survey in 2012 – most likely brought to the surface by recent plowing – and were immediately drawn to its distinctive polychromatic appearance. About 780 glass fragments were collected at the same time, but these had the usual iridescent patinas common to ancient Roman glass. This shard, while mostly dark green in color, was covered in a millimeter thick patina of a golden hue that was almost mirror-like in its reflective properties. To learn more, Omenetto and his co-authors subjected the shard to optical microscopy and a new type of scanning electron microscopy (SEM), which reveals not only the structure of the material at nanometer resolution, but also its elemental composition.
Chemical analysis dated the glass to between the first century BC and the first century AD. There was a high amount of titanium, indicating that the sand used to make the glass was of Egyptian origin, which usually had more impurities. As for the dark green color that is still present in the volume of the fragment, the authors assume that this is due to the presence of iron. Until about the middle of the second century AD, Roman glass was made either from Syro-Levantian raw glass made from relatively pure sand – resulting in a blackish-purple color – or from high-magnesium glass made from impure iron-rich sand with the addition of vegetable ash to give it a dark green color. This is consistent with this new “wow glass” analysis.
SEM analysis showed the presence of a precise hierarchical arrangement to form so-called “Bragg beams” – essentially one-dimensional photonic crystals with alternating layers of high- and low-refractive-index materials that lead to structural color. In an ideal Bragg stack, the layers have the same thickness. But one layer was thicker and denser than the other in “wow glass”, giving it that shiny metallic look. Specifically, each Bragg beam reflected a different narrow wavelength of light, and dozens of them stacked together resulted in a highly reflective golden patina on the shard.
This is evidence that the glass artifact was formed by a “pH-controlled chemical alteration of silica that does not impose the same strict material constraints as in natural animal-based systems,” the authors wrote. According to to Omenetto, if they could figure out a way to speed up the process so that such a patina didn’t take centuries to form, “we might find a way to grow optical materials rather than manufacture them.”
“It’s probably a process of corrosion and reconstruction,” said co-author Giulia Guidetti, also at Tufts. “The surrounding clay and rain determined the diffusion of minerals and the cyclic corrosion of silica in the glass. At the same time, the cyclic assembly of layers with a thickness of 100 nanometers combining silicon dioxide and minerals took place. The result is an incredibly ordered arrangement of hundreds of layers of crystalline material. The crystals grown on the surface of the glass are also a reflection of the changing conditions that occurred in the land as the city developed – a record of its environmental history.
PNAS, 2023. DOI: 10.1073/pnas.2311583120 (About DOI).